Galveston, U.S. temperatures in 2012 were 3-in-10,000-year events

Note: The following is a guest post by a regular reader and commenter, JohnD. You can find more of his work on his own blog, Factismals. He also performed a highly regarded review of the city’s record August, 2011 temperatures. Below, to see larger versions of the images, just click on them.

As we’ve done before, let’s spend a moment reviewing the basics before we investigate the main question: Was 2012 unusually hot?

The use of the average (a.k.a., the arithmetic mean) is fairly well-known, as is the idea of the mode and the median. And most of you know by now that a Gaussian normal curve (or bell curve, for the shape it makes) is the standard workhorse in statistics as it allows scientists to quickly estimate the odds of something happening by chance. You also know that when the skewness (distortion toward one side or the other) gets above about +/-0.5, then a Gaussian curve is no longer a good approximation. Similarly, the distribution may be flat (“have a negative kurtosis”) or very sharp (“have a positive kurtosis”); if the normalized kurtosis is above +/-1.5, then a Gaussian curve is no longer a good approximation. As always, I mention these rules of thumb so that you can decide for yourself if the data is good.

I’ve extracted the temperature data from two places. The Annual Mean Temperatures for the contiguous United States were taken from the NOAA web page. And the Monthly Mean Temperatures for Houston and Galveston were downloaded from the National Climactic Data server; the Galveston records were supplemented by Charles Roeseler of the Houston/Galveston office of the National Weather Service. Annual Mean Temperatures were calculated using the arithmetic average of the previous twelve months of Monthly Mean Temperatures.

Please allow me to digress briefly before we get into the analysis proper. One common concern about temperatures in general and those for Houston in specific is that the location where the temperature was taken has moved several times over the past 117 years. A related concern is that some of the locations are closer to the center of the city and so may by biased by the Urban Heat Island effect. In order to explore these questions, I plotted the Monthly Mean Temperatures for several locations around Houston over the past 93 years. As the plot shows, none of the areas is suspiciously anomalous; all of the values are very close together. This implies that the Urban Heat Island effect is not affecting the Houston records and that the changes of location have had minimal effect.

Houston monthly mean temperatures.

So what happens when we look at the Annual Mean Temperature for Houston, Galveston, and the Contiguous United States? Was last year truly anomalous (statistically speaking) or was it just hot? We’ll start by looking at the time series.

Looking at the plot of Annual Mean Temperatures for each of the regions, the difference between local weather and broad-scale climate becomes obvious. Houston suffered an exceptionally cool spell during the late 70s to early 80s, but Texas as a whole did not. In addition, the southern location of Houston and Galveston makes them nearly five degrees warmer on average than Texas as a whole, and more than 18 degrees warmer than the contiguous United States.

Annual mean temperature.

This becomes even more obvious if we look at the five-year means (i.e., the average of the previous five years’ worth of data). The spikes caused by short-term events are smoothed out by this technique and the overall long-term signal becomes even clearer. When temperatures in the contiguous United States trend upward, those in Houston and Galveston have larger swings and no clear pattern emerges. So local conditions clearly play a strong role in transforming climate into weather and make it harder to see long-term climactic shifts; put simply, if you want to see climate, you need to look over a large area and for a long time. Single-year records in single locations are not a reliable gauge of climate change.

Five year mean annual temperatures.

So, with that caveat in mind, how unusual was last year? As always, we start by looking at the mean, standard deviation, skewness, and kurtosis for the data. The low skewness and kurtosis indicate that a Gaussian curve is a good approximation to the data. And that is what we see in the bar charts as well; in general, the data (blue bars) is fairly close to what would be expected for a Gaussian curve (red bars).

And that is what we see in the bar charts as well; in general, the data (blue bars) is fairly close to what would be expected for a Gaussian curve (red bars).

But what is truly interesting is how last year plots on the charts. For Houston, last year was the warmest but it is just barely outside of the 2-sigma limit; it was hardly worth noticing. The same is true for Texas as a whole; last year was exceptionally hot but not as mind-boggling as August of 2011 was. However, for both Galveston and the contiguous United States as a whole, 2012 was freakishly hot. We would expect to see a year that warm for the contiguous United States or Galveston about three times in ten thousand years. But we’d see one that warm every seventy years for Houston and every 130 years for Texas as a whole.

So, what can we conclude from all of this?

Local one-year records say very little about long-term, broad-scale climate

Houston’s strongly variable weather makes it very hard to see any climate signal

Last year was absurdly hot in some locations (e.g., Galveston) and for the contiguous US as a whole, but hardly worth noticing here in Houston.